Semiconductor devices have become complicated multi-layered structures. Further, semiconductor devices have become highly integrated. Thus, reducing the electrical resistance of conductive patterns such as gate electrodes has become desirable.
Generally, a polysilicon layer pattern is employed in a gate electrode. To decrease electrical resistance of a gate electrode, a metal silicide layer pattern is formed on the polysilicon layer pattern. To reduce the electrical resistance of a gate electrode, a tungsten layer pattern is conventionally formed on the polysilicon layer pattern.
A conventional method of forming a gate electrode includes sequentially stacking a tungsten layer pattern and a polysilicon layer pattern.
For example, a polysilicon layer and a tungsten layer are sequentially formed on a semiconductor substrate. The polysilicon layer and the tungsten layer are partially etched to form a polysilicon layer pattern and a tungsten layer pattern.
However, when the etching process is performed, etching damage may be generated at surface portions of the substrate, the polysilicon layer pattern and the tungsten layer pattern due to plasma used in the etching process. Thus, an additional oxidation process (sometimes referred to as a re-oxidation process) for curing the etching damage is performed on the substrate, the polysilicon pattern and the tungsten pattern.
The re-oxidation process is performed in a furnace or rapid thermal processing (RTP) equipment at a temperature of no less than about 800° C. under an oxygen atmosphere or a water vapor atmosphere. When the re-oxidation process is performed at the above-mentioned high temperature, etching damage is sufficiently cured. On the contrary, a gate oxide layer may have a greater thickness than desired due to an excessive bird's beak. The term “bird's beak” is well understood by those skilled in the art and need not be explained further. This is referred to as a punch-through phenomenon of a gate oxide layer. The punch-through phenomenon may cause variable threshold voltage of a transistor or deteriorating reliability of the gate oxide layer.
To overcome the above-mentioned problems, a radical oxidation process that is performed at a relatively low temperature has been proposed to replace the re-oxidation process. Although the radical oxidation process may improve the punch-through phenomenon of the gate oxide layer, efficiency for curing the etching damage may be reduced and the gate electrode may not have a sufficiently rounded edge.
The re-oxidation process is performed under a condition that tungsten in the gate electrode is not oxidized. However, it is very difficult to prevent the oxidation of tungsten in the gate electrode. In particular, when the gate electrode is oxidized by the radical oxidation process, a sufficient amount of oxygen is provided to the gate electrode to round the edge of the gate electrode. However, when a flux of oxygen is increased, tungsten in the gate electrode is oxidized to form tungsten oxide (WOx) on a surface of the tungsten layer pattern.
When a heat budget is applied to the tungsten oxide material, needle-shaped whiskers grow. Because the whiskers include a conductive material, a short between wirings may be generated due to the whiskers. As a result, a semiconductor device incorporating the gate electrode may operate abnormally.
Examples of methods for removing the tungsten oxide material are disclosed in Korean Patent Laid-Open Publication Nos. 2001-0039009 and 2003-0080239.
However, an oxidation process and a process for removing the tungsten oxide material disclosed in the above-mentioned Publications are performed at a relatively high temperature of no less than about 800° C. Thus, a bird's beak is generated in the gate oxide layer so that the punch-through phenomenon of the gate oxide layer occurs.